Seems to be lots of conjecture on the squawk code leading to a theory which is IMHO a total red herring.
First off, at the time of the accident, Aldergrove was not SSR equipped and therefore would not have observed any code. They would only have assigned one if the next SSR equipped agency had passed the required code to them. Otherwise, the pilot of the Chinook would have selected an appropriate conspicuity code in accordance with SOPs. (The code 7000 would be appropriate for this flight). In this accident, the only possible unit which could have done so would have been Scottish Area Control Centre (either Civil or Military). For low level VFR flight, it is not normal practice to assign a specific code unless the pilot requests a radar service. Additionally, the nearest radar sources available to ScACC are at Tiree or Lowther Hill, both of which would be very hard pushed to pick up a low level target in the accident area due to terrain shielding and line of sight restrictions. Otherwise the radar recordings would have been used to provide a lot more evidence of ground track and altitude in the accident enquiries.
Coming to the SSR codes, the UK Code Allocation Plan is a document in the public domain. The 77** series of codes, with the exception of 7700, are all in use in normal every day operations. The main bulk of the series is allocated to ATC agencies for inter European traffic, including the disputed code of 7760. There are no 'secret squirrel' codes in the group. ATC radar systems therefore do not assume anyone is on an emergency if they squawk 77**. It would be chaos in Europe if they did !!
7776 and 7777 are used for SSR monitors. These are low power transponders fixed in a location on the ground at a known distance and bearing from the SSR radar aerial which it serves. The SSR processing equipment has a tolerance box within which it expects to see the ground transponder. If it moves outwith this area, then the radar is potentially out of tolerance and warnings will be given to ATC engineering, and in some locations to the ATC controller on his/her screen. A pilot selecting one of these codes will also cause the same thing to happen since the chance of him remaining in a box of extremely small dimensions in a position unknown to him/her is improbable. These codes are nothing to do with clandestine DME units.
Another that suggests that 7760 has an interesting meaning is:
<< RRR 7750-7799 ANDOVER RAF SPECIAL TASKS>>
I think you will find this refers to R/T callsigns and not SSR codes. Transponders use octal code and the highest digit which can be displayed is 7, therefore 7799 is impossible to set. It's good fun getting a pilot to try occasionally
